Partially crystallizing glass enamel compositions that fuse at relatively low temperatures are used, for example, to form opaque dark-colored enamel bands on the outer edges of sections of automotive glass such as windshields and side and rear windows. These opaque dark-colored enamel bands, which typically vary in width from about 1.5 cm to about 15.0 cm, greatly enhance the aesthetic appearance of the sections of glass upon which they are applied and also block the transmission of sunlight through the glass to protect underlying adhesives from degradation by ultraviolet radiation. Moreover, these opaque colored enamel bands preferably have the ability to conceal silver-containing buss bars and wiring connections of rear glass defrosting systems from view from the outside of the vehicle.
As noted in Gettys et al., U.S. Pat. No. 4,882,301, glass sections for automotive applications are often produced with varying degrees of curvature as opposed to flat, planar surfaces. If a curvature is desired in a given section of glass, the glass is heated to a temperature in the vicinity of about 700° C. and then subjected to a bending or curving stress employing any number of suitable molding or pressing techniques. At or near that temperature, the section of glass can be bent as desired, and the surface of the section of glass will maintain sufficient stiffness to resist marking or defects caused by contact with the press head of the equipment employed to bend or curve the glass or the vacuum head utilized to pick up and transport the section of glass during the bending operation.
It was discovered several years ago that specially formulated glass enamel compositions could be applied to planar sections of glass and fired to form opaque dark-colored enamel bands at the same time as the bending or forming operations were performed on the section of glass. As set forth in U.S. Pat. No. 4,882,301, these glass enamel compositions had to have the ability to fuse and partially crystallize at the temperature at which a section of glass would be preheated preparatory to a bending or forming operation. It is believed that the partial crystallization of the enamel forms a dense, hard, protective layer that prevents the enamel from sticking to the press or vacuum head during the glass bending and transporting operations.
Generally speaking, prior art glass enamel systems suitable for use in such automotive applications fit within one of five broad categories or types. The first category relates to lead and/or cadmium based enamel systems that partially crystallize upon firing. Glass enamel systems such as disclosed in U.S. Pat. No. 4,882,301 are representative of this type.
The second category relates to lead-free and cadmium-free enamel systems that include crystalline seed materials that promote partial crystallization of the enamel upon firing. Glass enamel systems such as disclosed in Ruderer et al., U.S. Pat. No. 5,153,150, Ruderer et al., U.S. Pat. No. 5,208,191, Sakoske, U.S. Pat. No. 5,677,251, Sakoske et al., U.S. Pat. No. 5,714,420, Sakoske, U.S. Pat. No. 5,753,685, and Sakoske, U.S. Pat. No. 5,783,507, are representative of this type.
The third category relates to partially crystallizing lead-free and cadmium-free enamel systems that include substantial amounts of Bi2O3, but little if any ZnO. Glass enamel systems such as disclosed in Murkens, U.S. Pat. No. 5,203,902, and Manabe et al., U.S. Pat. No. 5,578,533, and Sridharan et al., U.S. Pat. No. 6,105,394, are representative of this type.
The fourth category relates to partially crystallizing lead-free and cadmium-free enamel systems that include substantial amounts of ZnO, but little Bi2O3. Glass enamel systems such as disclosed in Ruderer et al., U.S. Pat. No. 5,306,674, Anquetil et al., U.S. Pat. No. 5,350,718, Emlemdi et al., U.S. Pat. No. 5,504,045, Heitmann et al., U.S. Pat. No. 5,707,909, and Harada et al., U.S. Pat. No. 5,817,586, are representative of this type.
The fifth category relates to partially crystallizing lead-free and cadmium-free enamel systems that include both Bi2O3 and ZnO as essential components. Glass enamel systems such as disclosed in Roberts, U.S. Pat. No. 5,252,521, Ryan, U.S. Pat. No. 5,616,417, and Punchak, U.S. Pat. No. 5,629,247, are representative of this type.
Although improvements have been made in recent years, the chemical durability of known lead-free and cadmium-free glass enamel systems used in automotive glass applications has been less than desired. Therefore, a need exists for lead-free and cadmium-free enamel compositions that exhibit excellent chemical durability to acids, water, and alkalis. Such enamel compositions must be able to fuse and preferably, partially crystallize at temperatures at which sections of glass are preheated preparatory to forming operations so as not to stick to press or vacuum heads. Moreover, such enamel compositions should be effective in blocking ultraviolet radiation and in retarding the migration of silver and subsequent showing from overprinted buss bars and wiring connections of rear glass defrosting systems.